Acids, such as hydrochloric, sulfuric and carbonic, for stimulating the production from an oil well are well known. Conventionally these acids are liquids, which are injected into the oil well to react with rocks in the oil bearing strata and thereby increase the permeability of the strata. When this is done, the acids being pumped down the production tubing or casing will react with the material used to make such tubing or casing unless an inhibitor is mixed with the acid.
Accordingly, it is the primary object of the present invention to inject liquid carbon dioxide down the bore of an oil well to form carbonic acid with the connate water in the strata. The carbonic acid reacts with the rocks in the strata to increase permeability, but the likelihood of damage to the production tubing and casing of the well is inhibited because no acid is pumped therethrough.
The technique of forming acids in situ with the oil bearing strata in secondary and tertiary production wells, such as flooding are well known. Commonly, a gas is formed in the strata and the gas reacts with either drive water or connate water to form the acid. Some examples of these techniques are set forth in U.S. Pat. No. 2,001,350; 3,072,185,; 3,091,292; 3,259,187; 3,344,858; 3,353,597; 3,392,782; 3,398,791; and 3,532,165. However, no one to date has suggested injecting a liquid carbon dioxide down the bore of an oil well capable of primary production to form carbonic acid in the strata which reacts with rocks in the strata to thereby increase the permeability of the strata.
It is therefore an object of the present invention to provide a method of stimulating production from an oil well by injecting liquid carbon dioxide down the bore of the well to form carbonic acid with connate water in the strata which increases the permeability of the strata.
In accordance with the invention, a method of stimulating production from an oil well capable of primary production out of an oil bearing strata comprises the steps of injecting an anhydrous liquid including liquid carbon dioxide down the well bore having a casing at a pressure which enables gaseous carbon dioxide, formed by the evaporation of the liquid carbon dioxide, to permeate through the strata and react with connate water to make carbonic acid in the strata outside the casing. The well is then shut in for a period of time sufficient for the carbonic acid to react with rocks in the strata which thereby increases the permeability of the strata and stimulates production from the oil well.